1. Field of the Invention
The present invention concerns the bonding of a copper sheet to a substrate made of an electrically insulating material.
The present invention can be applied, notably, to the making of electronic power modules using a substrate made of electrically insulating material chosen from among alumina and aluminum nitride.
The making of electronic power modules requires the use of materials with high galvanic insulation and thermal conduction properties. Furthermore, the materials used should have an expansion coefficient close to that of the semiconducting materials used such as silicon. Moreover, it is clearly desirable for the materials forming the above-mentioned modules to be as inexpensive as possible.
2. Description of the Prior Art
Numerous processes have already been proposed for bonding a copper sheet to a substrate made of an electrically insulating material.
The documents U.S. Pat. No. 2,517,248 and GB- A-584 931 teach, for example, a method for bonding a metal sheet to a substrate, made of electrically insulating material, by using a soldering material placed between the metal sheet and the substrate. However, the formation of a layer based on soldering material between the metal sheet and the substrate does not give sets with the physical characteristics (expansion coefficient and thermal conductivity) required for the designing of electronic power modules.
The documents U.S. Pat. No. 3,517,432, CH-A 202 198 and GB- A-584 931 teach, for example, a method for bonding a metal sheet to a substrate, made of electrically insulating material, by using a soldering material placed between the metal sheet and the substrate. However, the formation of a layer based on soldering material between the metal sheet and the substrate does not give sets with the physical characteristics (expansion coefficient and thermal conductivity) required for the designing of electronic power modules.
The documents U.S. Pat. No. 3,517,432, CH-A 202 198 and GB-A-761 045 teach a methods for bonding a metal sheet to a substrate by heating the material above its melting point in order to obtain a diffusion of the metal in the substrate and, thereby, a mechanical bonding between these elements. It is noted, however, that, after melting, the metal generally loses its integrity and tends to break up into metallic droplets The result thereof is that the metal/substrate set does not have the physical characteristics required to make electronic power modules.
The documents EP-A-0 097 944 et EP-A-0 123 212 teach the making of a bond of a metal sheet with a substrate made of electrically insulating material through the initial formation of a bonding layer, for example by heat treatment, on the electrically insulating substrate. The sets thus obtained do not have the requisite physical characteristics for making electronic power modules.
It has, furthermore, been proposed to bond a copper sheet with a substrate made of an electrically insulating material by the formation of a eutectic material.
Thus, the document FR-A-2 181 049 teaches the placing of a copper sheet on a substrate made of an electrically insulating material, the heating of this set under a reactive gas atmosphere at a temperature ranging between the temperature for forming a eutectic mixture and the melting temperature of copper, and then the cooling of the set. The documents U.S. Pat. Nos. 3,994,430, 3,911,553, DE-A-3 036 128 and DE-A-3 204 167 propose a pre-oxidation of the metal sheet before placing it on the electrically insulating material and the heating of the set thus obtained under a controlled atmosphere at a temperature ranging between the temperature for forming a eutectic mixture and the temperature for the melting of copper.
The processes proposed until now for bonding a metal sheet to a substrate made of an electrically insulating material, with the formation of a eutectic mixture, have been very useful. However, these processes are not entirely satisfactory. For, a detailed analysis reveals that there are air bubbles present at the metal/substrate interface. The presence of these air bubbles is a local obstacle to any mechanical bonding between the metal and the substrate. Furthermore, these air bubbles enable diffusion, with adverse effects, of etching agents between the metal and the insulating substrate when metallic pads, in a pattern geared to a specific purpose, are obtained by elimination.
It has been attempted to remove the air bubbles thus revealed by making grooves, used as discharge channels, in the surface of the metallic sheet as described, for example, in the document DE-A-33 24 661. It would seem that grooves of this type effectively make it possible to limit the formation of air bubbles at the metal/substrate interface. However, the above-mentioned grooves favor the harmful diffusion of etching agents between the metal sheet and the substrate.
In conclusion, hitherto proposed techniques for bonding a copper sheet to a substrate made of electrically insulating material are not entirely satisfactory.
After numerous essays and a great deal of research, the Applicant has determined that the pre-oxidation of the metal sheet, hitherto acknowledged as being indispensable before obtaining a temperature greater than the temperature for the formation of a eutectic mixture (pre-oxidation obtained through the application of a reactive atmosphere during the rise in temperature according to the document FR-A-2 181 049 or obtained by controlled pre-oxidation before heat treatment according to the documents U.S. Pat. Nos. 3,994,930, 3,991,553, DE-A-3 036 128 and DE-A-3 204 167) was in fact responsible for the formation of the above-mentioned air bubbles. Furthermore, in going against these received ideas, the Applicant has determined, after a great deal of testing and research, that an entirely satisfactory copper/substrate bond, meeting the physical characteristics required to make electronic power modules, can be obtained without pre-oxidizing the metal sheet.